1. Field of the Disclosure
This invention relates to a segmented baffle system designed for use in a riser that is used in a fluid catalytic cracking system.
2. Description of the Background of the Disclosure
Fluid catalytic cracking (FCC) is a catalytic hydrocarbon conversion process accomplished by contacting heavier hydrocarbons in a fluidized reaction zone with a catalytic particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of highly carbonaceous material referred to as coke are deposited on the catalyst to provide coked or spent catalyst. Vaporous lighter products are separated from spent catalyst in a reactor vessel. Spent catalyst may be subjected to stripping with an inert gas such as steam to strip entrained hydrocarbonaceous gases from the spent catalyst. A high temperature regeneration with oxygen within a regeneration zone operation burns coke from the spent catalyst which may have been stripped. Various products may be produced from such a process, including a naphtha product and/or a light product such as propylene and/or ethylene.
The basic components of the FCC process include an internal or external riser, a reactor vessel in which spent catalyst is disengaged from product vapors, a regenerator, and a catalyst stripper. In the riser, the hydrocarbon feed contacts the catalyst and is cracked into a product stream containing lighter hydrocarbons. A steam or gas stream is used to accelerate catalyst in a first section of the riser before introduction of the feed. Regenerated catalyst and the hydrocarbon feed are transported upwardly in the riser by the expansion of the gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums, upon contact with the hot catalyst.
The structure of an external riser includes a terminal end that is located outside of the reactor vessel. A transport conduit at the end of the riser directs a mixture of product vapors and catalyst into the reactor vessel containing a number of cyclones for separating spent catalyst from the product stream. The transport conduit may exit into a cyclonic separator contained in the reactor to make a first rough separation of catalyst from product vapors. Conventional designs have incorporated a terminal cap at the end of a riser to reverse the flow of the catalyst and cracked product vapors. Such devices are primarily for the purpose of disengaging catalyst particles from the cracked product stream. These conventional designs involve internal risers with terminal ends located in the reactor vessel. The reversed flow mixture of catalyst and product vapors exit an open bottom end of the cap annular to the riser. The catalyst falls downwardly through the open bottom end into a lower catalyst bed while product vapors ascend from the open bottom end into the open volume of the reactor vessel to effect a rough separation.
The velocity of the catalyst as it travels through the riser in the FCC process is particularly important for the realization of operating parameters that are similar to that in an ideal plug flow reactor. Ideally the feed, catalyst, and product vapor mixture may move in a plug flow regime to get the best product selectivity. In a plug flow regime, the catalyst and hydrocarbon vapor are flowing at the same speed up the riser, thereby eliminating back mixing or catalyst slip. Back mixing of the catalyst or slippage in the riser can lead to less selective cracking of the heavy oil to less profitable very light hydrocarbon gas.
In particular, the vapor and catalyst in the riser have slow velocity distributions adjacent the walls of the riser, which indicates a high catalyst holdup and overcracking of the slower moving vapor. Conversely, the vapor and catalyst in the center riser has a fast velocity distribution in the center of the riser, which indicates a low catalyst holdup and under-conversion of the faster moving vapor. Therefore, it would be desirable to offer a way to provide a uniform velocity profile for the vapor and catalyst.
One such way to adjust the velocity profile of the catalyst in the desired manner is through the use of one or more baffles. However, implementing baffles in risers in a retrofit situation is particularly problematic in that the existing refractory must be cut and erosion must be controlled downstream of the baffles. Therefore, it would be desirable to provide a way to retrofit risers using a segmented baffle system that does not require substantial reconstruction or repair of the existing riser and protects the riser from eddies.